Medicament delivery devices such as injectors have become widespread on the market, and in particular injectors primarily intended for self-administration of medicament. In that context, the injectors should be easy and almost intuitive to handle and use, and therefore a number of automatic or semi-automatic features are built in.
One example of such an injector is disclosed in patent application No. EP 1 743 666. The injector is arranged with a number of automatic features where the most important is the auto-injection feature. This is solved according to the document in that a plunger rod, which can act on a container for expelling medicament, is arranged with an annular groove. Flexible tongues are placed in the groove for locking the position of the plunger rod. In turn, the flexible tongues are held in place by a locking ring surrounding the tongues. When the injection is performed the locking ring is moved axially somewhat, and also the tongues, whereby the locking ring releases the tongues, which in turn causes the tongues to flex out of the groove of the plunger rod. The plunger rod is now free to move and is pushed towards the medicament container due to a drive spring and the injection is performed until the stopper inside the medicament container reaches its foremost position, in which the dose delivery is completed.
For some medicament rather large dose quantities are needed to be injected, often also with rather large needles. These large doses can be rather painful for the patient when they are injected into the tissue in one continuous movement, as with the injector described in EP 1 743 666. There are also some medicament types that as such cause pain when in contact with the tissue. When self-administering, there could thus be a risk that the patient withdraws the injector due to the pain or discomfort, before the whole dose is delivered. It would thus be desirable to temporarily stop the injection a number of times and allow the injected part of the dose to spread into the tissue before continuing the injection.
Document U.S. Pat. No. 5,679,111 discloses an injection device comprising an auto injection mechanism comprising a plunger rod operated by an injection spring for expelling a preset dose of medicament and a user controlled releasable plunger rod locking mechanism. The latter comprises a longitudinal groove with a wedge-shaped cross-section and a locking lug with cooperating cross-section. The locking lug is urged into the groove for locking the movement of the plunger rod. The locking lug can be moved out of contact with the groove by a user-operated handle whereby the plunger rod is moved, until the handle is again released or that the plunger rod comes to a dose stop. Thus the user is capable of controlling the injection sequence via the handle/lug in and out of frictional contact with the groove of the plunger rod.
A drawback with the device according to '111 is the use of frictional forces for holding/locking said plunger rod. A person skilled in the art is well aware of that the force from an injection spring needs to be rather high, in particular at the start of the injection sequence for overcoming the initial forces for moving a stopper inside the medicament container. It is thus not certain that the friction forces are capable of withstanding the forces from the injection spring.
Also, when the plunger rod is moving with a certain speed during injection, it might be impossible to stop the movement by mere friction. It is thus doubtful as to whether frictional locking mechanisms are capable of providing the user with real possibility of temporarily stopping an injection sequence as mentioned above.
There is thus room for improvements regarding the possibilities for a user to control an injection sequence.